The background of the invention will be set forth in two parts.
1. FIELD OF THE INVENTION
This invention relates to delay lines and more specifically to bulk acoustic wave delay lines.
2. DESCRIPTION OF THE PRIOR ART
The usefulness of propagating elastic wave energy in solids has been known for many years and is generally known as microwave acoustic technology. Utilizing this technology, such devices which store and delay signals have been developed to a relatively high degree. Many texts are presently available which thoroughly describe the history and advancements of this art, such as, for example, "Ultrasonic Methods In Solid State Physics" by Rohn Truell, Charles Elbaum and Bruce B. Chick, Academic Press, 1969.
Probably of greatest interest in the field of bulk wave devices has been the bulk acoustic wave delay lines. Unlike surface acoustic wave delay lines in which most of the energy propagating along an elastic surface is converted to electromagnetic wave energy upon reaching a state of the art transducer, only about 10% of the propagating bulk wave energy is converted at an output transducer, the rest being reflected back toward the input transducer. This relatively strong reflected wave is again reflected, at the input transducer, and is incident on the output transducer to produce a relatively strong spurious signal known generally as the triple-transit signal.
Although there was at first much interest in bulk acoustic wave devices because they are more adaptable for operation in the multi-gigahertz range as compared to surface acoustic wave devices (usually limited to about 500 MHz), the problem of the triple transit signal has caused a decrease in such interest. The problem of triple transit signals has become more severe as techniques were developed which reduced the attenuation per unit time of propagating acoustic wave energy so as to increase its relatively narrow bandwidth characteristic, as will be described hereinafter.
Attenuation is required in order to reduce this echo problem because it more greatly affects the multiple path triple transit echo signal than the single transit main signal. However, the increase of attenuation per unit time of the propagating energy in the solid is extremely frequency dependent. That is, the attenuation .alpha. in dB per microsecond is proportional to f.sup.2, where f is the frequency of the propagating acoustic wave energy. Thus a 5 dB attenuation of this type at 5 GHz will be approximately 20 dB at 10 GHz. This f.sup.2 dependence is more completely discussed in such articles as "On the Absorption of Sound in Solids"by A. Akhieser in The Journal of Physics (USSR), Vol. 1, Page 277 (1939), and in "Absorption of Sound in Insulators" by T. O. Woodruff and H. Ehrenreich, in Physical Review, Vol. 123, page 1553 (1961).
Several techniques have been developed in order to lessen the overall frequency dependent attenuation and thereby increase the bandwidth of the device. The lowest presently known room temperature attenuation at 10 GHz occurs in magnesium aluminum spinel (MgA1.sub.2 O.sub.4), which is .alpha. = 8 dB/usec. Through cooling the solid to very low temperatures of about one-fifth of its Debye temperature (about 180.degree.K), this attenuation is reduced to a much lower level, and this greatly improves the bandwidth. However, the triple transit signal is no longer suppressed. It has been found that the main delayed output signal and the triple transit output signal for a double ended bulk delay line, as shown in FIG. 1, is about 2.alpha.L in dB, where L is the length of the path between transducers.
For a more complete discussion of the affect of lowered temperatures on the attenuation of propagating acoustic waves in elastic solids, reference may be made to an article entitled "Temperature Dependence on Microwave Phonon Attenuation" by M. Pomerantz in Physical Review, Vol. 139, A501 (1965) at page 22.
Another technique which has been developed in an effort to reduce the overall attenuation is to utilize mode converting surfaces which convert a longitudinal or compressional bulk wave to a shear or transverse bulk wave. This technique uses the well-known principle that longitudinal waves can be converted to shear waves by reflection off of a free surface disposed at a predetermined angle with respect to the incident wave. Delay lines utilizing this scheme use a transducer to generate and receive the longitudinal waves while the substantial part of the propagating acoustic wave energy traverses the device in the shear wave mode. This scheme, along with the cooling technique, substantially reduces this f.sup.2 bandwidth problem but, as mentioned before, increases the triple transit problem. It should therefore be evident that a new technique which will maintain a relatively wide bandwidth characteristic in a bulk delay line while providing a relatively high triple transit signal suppression would constitute a significant advancement in the art.